Well, it turns out I solved this one with a little clarification.
The two packages that I was using were based on Red Hat 7. I am not completely sure if it was the issue, but here is what I did.
I found out that Centos is based on Red Hat Enterprise Server 4. I then went in search of an rpm of SpamAssassin already built for rhel4 and found one. I installed using rpm -ivh /directory/***packagename.rpm and got the same error. (Which was a dependancy error)
Then I went out and found an rpm package for Digest SHA1 and a few others that were required by Digest. I installed them in order of requirement using rpm and all went well.